This invention relates to an ion beam generator for an ion implantation system which permits the implanting of ions into substrates such as semiconductor wafers. More particularly, the invention relates to improvements in the extraction electrode assembly for such an ion beam generator.
Ion implantation techniques for modifying the electrical conductivity properties of semiconductor materials are well known. In order to generate the ions necessary for implantation into the semiconductor wafer, an ion source is provided which generates ions of a chosen element. An extraction assembly comprising a plurality of electrodes is provided downstream of the ion source, to extract, accelerate and focus the ions before they enter a mass analyser and selector.
The traditional arrangement of electrodes in the extraction assembly is a so-called triode structure with three electrodes. More recently, a tetrode structure has been proposed with four sets of electrodes. This is set out in WO 99/23685.
There are several considerations in the extraction of ions from an ion source, such as beam tuning to optimise extraction efficiency, focussing of the beam and so forth. The present invention seeks to improve these and other factors.
According to a first aspect of the present invention, there is provided an ion beam generator for an ion implantation system, the ion beam generator comprising: an ion source for generating ions to be implanted; and an extraction electrode assembly comprising a plurality of electrodes for extracting and accelerating ions from the ion source; the extraction assembly-including a source electrode at the potential of the ion source, an extraction electrode adjacent to the source electrode to extract ions from the ion source along a first ion extraction axis, and a ground electrode downstream of the extract ion electrode in the said ion extraction direction; wherein the source electrode and the extraction electrode each have a curved portion adjacent an electrode aperture, the curvature of each curved portion being in a first plane defined between the ion extraction axis and a second axis orthogonal thereto, the distance radially from a surface of the curved portion of the source electrodes to a surface of the curved portion of the extraction electrode being constant along at least a part of the length of each curved portion.
With prior art ion beam generators, the source and extraction electrodes both have the same radius of curvature. This means that, whilst the separation of the surfaces remains constant in a direction measured parallel to the ion extraction axis, the radial separation between the surfaces reduces from a maximum along the ion extraction axis to a minimum at the outer extremities of the source and extraction electrodes.
By contrast, by forming both the extraction and source electrode such that their separation in a radial direction is constant, the electrodes remain equidistant in the direction in which they focus a beam of ions exiting the ion source (i.e. towards the centre of their curvatures). With the arrangement of concentric electrodes provided by the present invention, an increase in ion beam current is obtainable.
The extraction electrode assembly may be either a triode structure, in which case the extraction electrode also acts as a suppression electrode, or may instead, and in the preferred embodiment, be a tetrode structure with a separate suppression electrode provided between the extraction electrode and the ground electrode. Most preferably, the curved portions of the source and extraction electrodes are part-cylindrical. In that case, a section through these curved portions will each define a part of a circle of different radius but with a common centre.
In accordance with a second aspect of the present invention, there is provided an ion beam generator for an ion implantation system, the ion beam generator comprising: an ion source for generating ions to be implanted; and an extraction electrode assembly comprising a plurality of electrodes for extracting and accelerating ions from the ion source; the extraction assembly including (a) a source electrode at the potential of the ion source; (b) an extraction electrode adjacent to the source electrode and at a potential different thereto, to extract ions along a first ion extraction axis; (c) a suppression electrode downstream of the extraction electrode along the ion extraction axis; and d) a ground electrode downstream of the suppression electrode and maintained in use at a ground potential; the source electrode and the extraction electrode being relatively movable in a direction generally parallel with the said first ion extraction axis.
Allowing a variation in the extraction gap between the source electrode and the extraction electrode permits the extracted beam current to be increased when necessary by decreasing the gap. When less beam current is required, the gap can be extended with the advantage of minimising the possibility of high voltage breakdown between the electrodes. The variable gap also provides the option of being able to modify the plasma boundary shape and, thereby, help tune the beam through the ion implanter.
According to a further aspect of the present invention, there is provided an ion beam generator for an ion implantation system, the ion beam generator comprising: an ion source for generating ions to be implanted; and an extraction electrode assembly comprising a plurality of electrodes for extracting and accelerating ions from the ion source; the extraction assembly including a source electrode at the potential of the ion source, an extraction electrode adjacent to the source electrode to extract ions from the ion source along a first extraction axis, and a ground electrode downstream of the extraction electrode in the said ion extraction direction; wherein the ion source includes an ion source front plate having a first part relatively distant from the ion extraction axis and a second part relatively adjacent to the ion extraction axis, the said first part being offset from the second part along the said ion extraction axis and wherein the said first part is downstream of the said second part such that the said second part forms a re-entrant surface.
Typically, the ion source of an ion beam generator for an ion implantation system employs either a directly or an indirectly heated cathode (IHC). For example, the directly heated Freeman and Bernas sources are well known. The internal structure of such sources requires that the front plate of the ion source is displaced, along the ion extraction axis.
By forming a re-entrant in the front face of the ion source, an aperture formed in the source electrode moves further toward the plasma generation region which in turn potentially allows more of the ion beam to be extracted. Furthermore, the re-entrant itself may be curved (i.e. concave) and this may improve beam focussing.
In still a further aspect of the present invention, there is provided an ion beam generator for an ion implantation system, the ion beam generator comprising: an ion source for generating ions to be implanted; and an extraction electrode assembly comprising a plurality of electrodes for extracting and accelerating ions from the ion source; the extraction assembly including (a) a source electrode at the potential of the ion source; (b) an extraction electrode adjacent to the source electrode and at a potential different thereto, to extract ions along a first ion extraction axis; and (c) an electrostatic lens comprising a lens electrode and a ground electrode, the lens being arranged downstream of the extraction electrode so as to control the focussing of ions there.
The use of an electrostatic lens provides more controllable focussing in both the horizontal and vertical plane, that is, in the two planes defined between the ion extraction axis and a first axis orthogonal thereto, and the ion extraction axis and the other axis orthogonal thereto, respectively.
In a preferred embodiment of an electrostatic lens, the lens electrode and ground electrode are concentric but axially offset cylinders, and in particular circular section cylinders. The ground electrode provides the inner cylinder and preferably includes a plurality of axially extending, radially displaced, slots. In a preferred embodiment, four slots may be provided which produce a quadrupole field, although dipole or other fields may be produced by, for example, altering the number of radially displaced slots. The electric field formed between the high tension outer cylinder (the lens electrode) and the grounded inner electrode (the ground electrode) penetrates the slots in the ground electrode so as to produce an electrostatic field.
In a particularly preferred embodiment, the extraction electrode assembly is a tetrode structure and further comprises a suppression electrode downstream of the extraction electrode along the extraction axis. It is of course to be understood that the lens can equally be employed in a triode structure.